Focal adhesion plaques are specialized regions of the plasma membrane through which cells in culture adhere to the external substrate (Burridge, K. et al., Ann. Rev. Cell. Biol., 4:487-525 (1988); Burridge, K. and K. Fath, BioEssays, 10:104-108 (1989)). On their internal face these structures anchor actin stress fibres, which are important in determining cell shape. Similar, but less well-characterized structures have been implicated in attachment between neighboring cells and adherence to extracellular matrix in vivo. Oncogenic transformation is frequently accompanied by a less-adherent, rounded morphology resulting from reorganization of the cytoskeleton (Ben Ze'ev, A., Biochem. Biophys. Acta., 780:197-212 (1985; Felice, G. R. et al., Eur. J. Cell Biol., 52:47-49 (1990)). In Rous Sarcoma Virus (RSV) transformed cells, for instance, it has been postulated that a contributing factor is the aberrant phosphorylation by pp60.sup.v-scr of tyrosyl residues in key focal adhesion proteins (Burr, J. G. et al , Proc. Natl. Acad. Sci. USA, 77:3484-3488 (1980); Parsons, J. T. and M. J. Weber, Curr. Topics in Microbiol. and Immunol., 147:79-127 (1989)). Phosphotyrosine has also been detected in focal adhesions (Maher, P. A. et al., Proc. Natl. Acad. Sci. USA, 82:6576-6580 (1985)) and apical junctions (Takata, K. and S. J. Singer, J. Cell Biol., 106:1757-1764 (1988)) in nontransformed cells, raising the possibility that tyrosine phosphorylation at these sites may regulate normal cellular function. Such phosphorylation events must be tightly controlled and an understanding of the mechanism(s) involved would be very useful in furthering our understanding of control of normal and neoplastic cell growth.